Gallosilicates are important catalysts in the petroleum and chemical industries for the preparation of valuable organic intermediates. Particularly noted for their dehydrating and aromatizing properties, gallosilicates have been employed for the conversion of lower molecular weight alkanes and alkenes to higher aliphatics and, in particular, one- and two-ring aromatic compounds. Aromatic compounds, such as benzene, toluene and xylene isomers, are important starting materials for the production of synthetic fibers, polyesters and other plastics as well as octane-increasing substances in lead-free motor gasolines.
Structurally, gallosilicates and other zeolites belong to the tectosilicates (see, for example, "Kirk-Othmer Concise Encyclopedia of Chemical Technology", M. Grayson, ed., John Wiley & Sons, publisher, New York, 1985, pages 772-774, for a review of zeolites). Their structure consists of TO.sub.4 tetrahedra that are linked to each other by sharing of the oxygen atoms at the vertices. The nature of the T atoms is variable. Aside from the tetravalent silicon, trivalent atoms (such as aluminum or gallium) can be incorporated in the lattice framework. The tetrahedra form interlocking chains and layers and build up a defined interconnected system of channels and pores having opening widths of molecular dimensions. The opening widths of the channels and pores ultimately determine the accessibility to the inner cavity structure for materials of defined shape and form and thus confer molecular sieve-like properties to the porous bodies. Non-framework alkali or alkaline earth metal ions, present in zeolites after synthesis, readily undergo ion exchange with protons to produce effective, heterogeneous, acidic catalysts.
Methods for the synthesis of gallosilicates are described extensively in the technical and patent literature (see, for example, D. K. Simmons, J. Catal. 106: 287-291 (1987) and EP 0 130 013, published Feb. 1, 1985). In general, gallosilicates are prepared from aqueous, alkaline mixtures of reactive silicon dioxide and gallium(III) oxide by hydrothermal crystallization in the presence of alkali ions and quaternary ammonium compounds. Tetraalkylammonium salts, such as tetrapropylammonium bromide, are incorporated into the mixtures as templates to aid formation of the framework lattice.
The reported methods for producing crystalline gallosilicates, however, generally suffer from a variety of deficiencies which include: (1) use of large amounts of costly tetraalkylammonium salts which, during calcination of crude zeolite products, produce considerable amounts of pollutants and thus necessitate relatively expensive control equipment and (2) use of excessive reaction temperature which is in the range of 400.degree. to 564.degree. C. which is costly in terms of energy consumption. More important, gallosilicate crystals produced by the conventional methods have a highly variable particle size range which is an undesirable characteristic for their use as catalysts. The application of gallosilicates with a highly variable particle size in catalytic reactions which are controlled by the diffusion rates of products results in faster deactivation.
Furthermore, none of the reported methods produce crystalline gallosilicate having a silicon dioxide:gallium (III) oxide ratio equal or greater than 5. A lower silicon dioxide: gallium (III) oxide molar ratio, i.e. a higher proportion of gallium is desirable because the catalytic activity of the zeolite is directly proportional to the amount of gallium present in the lattice framework. As defined herein, catalytic activity is an inherent property of a zeolite catalyst to enhance the rate of conversion of low molecular weight hydrocarbons to higher aliphatics and aromatic compounds.
Accordingly, there is a substantial need in the field for improved methods for preparing crystalline gallosilicates which avoid at least one of the deficiencies mentioned above. Furthermore, there is an acute need in the art for gallosilicates with a silicon dioxide: gallium (III) oxide molar ratio equal or greater than 5.